Modern internal combustion engines are equipped with sophisticated systems to monitor and control various aspects of engine performance during ongoing operation to meet operator demands for performance, including torque and fuel economy, and to satisfy government regulations related to emissions, safety, and fuel economy. Engine systems operable at lean air/fuel ratios, including diesel engines and homogeneous-charge engines, are effective to achieve operator demands. Such engine systems utilize exhaust aftertreatment systems comprising diesel particulate filters (DPF), NOx traps (LNT), NOx adsorbers, and selective catalyst reduction (SCR) devices, either alone or in combination.
An exhaust aftertreatment device can require regeneration during its service life to maintain performance, and regeneration is often achieved by adjusting an exhaust gas feedstream to a stoichiometric or rich air/fuel ratio while operating at an elevated temperature, e.g., in the range of 600 C. The exhaust gas feedstream can be adjusted by controlling an engine management system, including injecting fuel into the combustion chamber post-combustion, modulating ratios of air and re-circulated exhaust gas, and other strategies including injection of a reductant into the exhaust gas feedstream upstream of the device.
Referring now to FIG. 1, data representing an in-cylinder pressure waveform is plotted as a function of engine crank angle in degrees (CA) for normal engine operation (FIG. 1A) and for engine operation which includes post-injection of fuel (PCI) (FIG. 1B). As shown by comparing the in-cylinder pressures in the figures, post-injection of fuel typically results in a second pressure spike and an increase in engine brake mean-effective-pressure. Thus, a system which injects fuel into the combustion chamber during a latter portion of the power stroke, i.e., post-combustion, can experience a torque boost during regeneration of the aftertreatment system. The torque boost results from burning of some of the post-injected fuel, and generates torque, perceptible as engine roughness.
One partial solution comprises balancing engine torque by tuning of main fuel injection and the post-injection of fuel through calibration on an engine dynamometer. However, such calibration requires extensive testing and analysis, and may not result in reduced engine roughness during real vehicle driving conditions due to the amount of fuel being injected post-combustion is typically adjusted according to real-time catalyst temperatures and inlet air-fuel ratio. Furthermore, the main injection typically causes a drop in fuel injection line pressure which may not have recovered at the time of post-injection of fuel following the main fuel injection. Therefore, torque contribution caused by post-injection may not be consistent with a predetermined calibration torque.
Therefore, there is a need for an engine control system which addresses the aforementioned concerns.